ACJC 2022 Prelim P3 QP

3 © ACJC2022 9729/03/Prelim/2022 [Turn over Section A Answer all the questions in this section. 1 (a) Explain what is meant by the term Lewis acid. [1] …………………………………………………………………………………………………… ………………………………………………………………………………………….............. …………………………………………………………………………………………………… (b) The water molecule can react in various ways: as an acid, as a base, as a nucleophile, as an oxidising agent and as a reducing agent. Study the following reactions and decide in which way water is reacting in each case. Explain your answers fully. (i) 2F2 + 2H2O → 4HF + O2 [1] (ii) CH3COCl + H2O → CH3COOH + HCl [1] (iii) H2PO4− + H2O HPO42− + H3O+ [1] (iv) Li + H2O → LiOH + ½H2 [1] (v) NO2− + H2O HNO2 + OH− [1] …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. ……………………………………………………………………………………………..

4 © ACJC 2022 9729/03/Prelim/2022 [Turn over …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. (c) The pKa values of three acids are listed in Table 1.1. Table 1.1 acid formula pKa 1 CH3CH2COOH 4.9 2 CH3CHClCOOH 2.8 3 CH2ClCH2COOH z (i) Explain the difference in pKa values between acid 1 and acid 2. [2] (ii) Suggest a value for z and explain your answer. [1] (iii) Peroxyacids are weak acids. One way to prepare peroxypropanoic acid is to treat the corresponding carboxylic acid with hydrogen peroxide. acid 1 peroxypropanoic acid Suggest why the pKa of peroxypropanoic acid is higher than that of acid 1. [1] …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. ……………………………………………………………………………………………..

5 © ACJC 2022 9729/03/Prelim/2022 [Turn over …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. (d) A, B and C are isomers with the molecular formula C 5H6O2. All three compounds decolourise bromine water in the dark. A produces effervescence in the presence of Na 2CO3(aq) whereas B and C do not. A also reacts with hot acidified KMnO4 to form D, C3H2O5. B forms a brick-red precipitate when heated with Fehling’s solution. C reacts with hot aqueous sodium hydroxide. Upon acidification, it forms C5H8O3. C5H8O3 When B and C are separately reacted with hot acidified KMnO 4, they form the same mixture of E, C3H4O4, and F, C2H2O4. F undergoes further oxidation to give effervescence. (i) Draw the structures of D, E and F. [3] (ii) Deduce the structures of A, B and C with reasoning. [7] …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. ……………………………………………………………………………………………..

6 © ACJC 2022 9729/03/Prelim/2022 [Turn over …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. [Total: 20]

7 © ACJC 2022 9729/03/Prelim/2022 [Turn over 2 (a) The acid -base behaviour of aluminium oxide, A l2O3, shows similarities to that of magnesium oxide, MgO, on the one hand, and sulfur trioxide, SO3, on the other. Describe what these similarities are, and explain why aluminium oxide occupies this in-between position. Write equations for all the reactions you choose to illustrate your answer. [5] ………………………………………………………………………………………………….. …………………………………………………………………………………………............. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. …………………………………………………………………………………………............. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. …………………………………………………………………………………………............. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. …………………………………………………………………………………………............. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. …………………………………………………………………………………………............. ………………………………………………………………………………………………….. (b) (i) Explain what is meant by the term standard enthalpy change of atomisation of magnesium. [1] (ii) An experiment was carried out by adding 0. 05 g of magnesium ribbon to